Sounds like two separate issues. Last one first. The cutoff valve is not closing completely. To replace it, you will have to shut the house supply valve. Second problem, tank bolts. These are available in plumbing supply houses. It try that before replacing. Be careful when tightening, don't put a bind on the tank by overtighening, especially just one side. It is possible your tank has a hairline crack, they are hard to see, but bolts are cheap.